1. Field of Invention
The present invention relates to a method for transmitting and receiving wireless local area network (LAN) data. More particularly, the present invention relates to a method and a system for transmitting and receiving wireless LAN data on the basis of time slots generated as a result of dividing a contention-free period into a plurality of time slots.
2. Description of the Related Art
Currently, there are several standards involved in data transmission in a wireless manner. For example, Bluetooth is used in constructing a small-sized wireless network, a third generation mobile communication in a WCDMA (wireless code division multiple access) mode, a wireless LAN based on IEEE 802.11 and the like. Among them, the wireless LAN using IEEE 802.11 standard has been successful and its use has increased.
The IEEE 802.11 Standard employs a technique using frequency hopping (FH) and a direct sequence spread spectrum (DS) and a technique using infrared (IR). The early IEEE 802.11 products were limited to 2 Mbps in their transmission speed, but their transmission speed is currently supported up to a maximum speed of 11 Mpbs according to the IEEE 802.11b Standard established in 1999. Meanwhile, 802.11a is based on a technique of orthogonal frequency division multiplexing (OFDM), operating in different frequency bands from 802.11b.
The 802.11 Standard comprises physical (PHY) layers using the frequency hopping and the direct sequence spread spectrum and using an orthogonal frequency division multiplexing. On the physical layer is positioned a data link layer comprising a sublayer of medium access control (MAC) based on 802.11 and a sublayer of logic link control (LLC) based on 802.2.
In a wireless LAN, there are four physical elements: an access point (AP), a wireless medium, a station and a distribution system. The AP performs a function of transforming a frame of a 802.11 network into a frame of a different shape so as to transmit it to a different network, namely a wireless-wired network bridging function. The wireless medium is a comprehensive concept comprising a radio frequency or an infrared physical layer to transmit a frame from a station to another station or an access point. The station refers to any device computing with a wireless network interface, for example, a notebook or a personal digital assistant (PDA). Last, the distribution system interconnects several access points so as to construct wider area covered thereby.
The network is based on basic service sets (BSSs), referring to groups of stations communicating with each other. The BSS is classified into an independent BSS allowing a station to directly communicate with other stations, and an infrastructure BSS compelling a station to communicate with other stations only through an access point. To compare both BSSs, the latter requires for a larger amount of transmission for communication than the former. However, the latter is more advantageous in that, when a station is entered into a power-saving mode, the access point can record it and do a frame buffering for the station, and it is not necessary to use complicated physical layers to maintain a relationship between movable stations since all the stations have to be located within an accessible range from the access point.
Under the 802.11 Standard, there are two methods of accessing media for data transmission: one is a contention based access and the other is a contention free based access. The former is referred to as a distributed coordinator function (DCF) and the latter as a point coordination function (PCF).
The PCF controls data transmission in a contention free period (CFP), thereby enabling data transmission without contention because an access point polls respective nodes sequentially as in a list of nodes, called as a polling list, in the CFP, with the polling list for data transmission without contention between stations. That is, only the station receiving a poll is authorized to transmit data. In this context, PCF can be called a polling and response protocol. In comparison with DCF, PCF is advantageous in that it can assure a quality of service (QoS) to some degree while a station is transmitting data.
FIG. 1 is an exemplary view illustrating data transmission in a contention free period.
Referring to this figure, if an AP polls for a station #1, the station #1 transmits data. The station #1 must pass through the AP in order to transmit data to a station #2. Then, the AP polls for the station #2. At this time, the AP transmits the data received from the station #1 to the station #2, along with the poll. The station #2 may send a null frame to the AP if it has no data to be transmitted. Subsequently, the AP polls for a station #3. The station #3 transmits the data to be transmitted to the station #1, to the AP. The AP polls for a station #4. The station #4 sends a null frame if it has no data to be transmitted. If the contention free period does not end, although the polls for all the stations are finished, the AP again polls for the station #1. At this time, the data to be transmitted to the station #1 by the station #3 is together sent to the station #1 with the polling.
Since this method assures all the stations that they are given an opportunity of transmitting data, but it causes too large overhead for polling. In a word, in order to allow a station to transmit data, an AP always has to poll for the station. Besides, the AP must poll for those stations having no data for transmission, such as station #2 and station #4, and the stations #2 and 4 having no data, have to send null frames. According to a conventional art, it is assured that all the stations are given an opportunity of transmitting data but a wireless channel is used in an inefficient manner.